7.2.1 Fiber’s Key Characteristics
The key characteristic of a fiber link is the power loss that is caused by fiber
attenuation and fiber connections. Attenuation, defined as the ratio of the input
power to the output power, is the loss of optical power as light travels along the
fiber. Attenuation in an optical fiber is caused by absorption, scattering, and
bending losses. The fundamental physical limits imposed on the fiber attenuation
are due to scattering of the silica atoms at shorter wavelengths and the material
absorption at longer wavelengths. There are two minima in the loss curve, one near
1:3 mm and an even lower one near 1: 55 m m. Fiber bending can also induce power
loss because radiation escapes through its bends. The bending loss is inversely
proportional to the bend radius and is wavelength-dependent. It is important to
mention that there may be many fiber macro bends during a PON installation, and
macro bends may have more significant losses at longer wavelengths.
Power loss is also present at fiber connections, such as connectors, splices, and
couplers. Coupling of light into and out of a small-core fiber is much more difficult
to achieve than coupling electrical signals in copper wires since: (i) photons are
weakly confined to the waveguide whereas electrons are tightly bound to the wire,
and (ii) the core of a fiber is typically much smaller than that of an electrical wire.
First, light must be coupled into the fiber from a diverging laser beam, and two fibers
must be connected to each other. Second, connecting two different fibers in a system
must be performed with great care due to the small size of the cores. One wishes to
achieve connections exhibiting: (i) low loss; (ii) low back reflection; (iii) repeatability;
and (iv) reliability. Two popular methods are the permanent splice and the mech-
anical connector. The permanent ‘‘fusion’’ splice can be accomplished by placing
two fiber ends near each other, generating a high-voltage electric arc which melts the
fiber ends, and ‘‘fusing’ the fibers together. Losses and back reflection are extremely
low being <0.1 dB and <60 dB respectively. Disadvantages are that: (i) a splice is
delicate and must be protected and (ii) it is permanent. Alternatively, there are
several types of mechanical connectors, such as ST, and FC/PC. Losses and back
reflection are still fairly good, and are typically <0.3 dB and <45 dB respectively.
Low loss is extremely important since a light pulse must contain a minimum
amount of power in order to be detected such that ‘‘0’’ or ‘‘1’’ data bit can be
unambiguously detected; thus optical power measurement (or fiber-link loss
measurement) is the most basic testing in any optical-fiber network. Note that
ORL characterization is related to optical power measurement, and is required for
many digital and analog fiber systems, such as PONs. ORL along a fiber span is a
combination of Rayleigh scattering and Fresnel reflections, which can reduce fiber
system performance and increase BER by degrading transmitter stability.
Network Testing, Characterization, and Monitoring Challenges for PON 269

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